Competition and complementarity in annual intercrops - the role of plant available nutrients

Summary

In the developed regions of the world crops are today primarily grown as sole crops and monocultures. However, increasing awareness of the link between agricultural practise, environmental issues and long-term stability of existing food production systems has put focus on the role that greater crop diversity in time (crop rotation) and space (field size and mixed cropping systems) may play in reducing the extent of these problems. Intercropping represents one way of increasing crop diversity. As a result of differences in the way component crops respond to and affect the environment in which they are grown intercrops may use available growth resources (light, water and nutrients) more efficiently, reduce the prevalence of disease and pests and reduce weed infestation compared to sole crops. The greatest intercrop advantages are attained when the species that are mixed differ markedly either morphologically, phenologically or physiologically. The mixture of a nitrogen fixing legume and a non legume is the most common intercrop combination and in Denmark this is primarily in the form of the pea (Pisum sativum) – barley (Hordeum vulgare) mixture.
In two field studies and one pot study the link between crop diversity, productivity and nutrient use was evaluated. The impact of crop density and the relative frequency of crop components in pea – barley intercrops was determined and the methods traditionally used to study the effects of intercropping compared to sole cropping were evaluated.
The mixture of a nitrogen fixing crop and a non fixing crop gave rise to greater productivity than in comparable sole crops. As a result of the legumes ability to use atmospheric nitrogen, an ability that was strengthened through competition from barley and rape for soil nitrogen, the intercrops displayed complementarity with respect to nitrogen use. Increasing the functional diversity of an intercrop, as a result of increasing the number of intercrop components from two to three did not give rise to greater yields or resource use.
The competitive dynamics at play between the component crops of an intercrop are to a large extent determined in the early growth phases. Barley was the fastest emerging crop component and thereby gained a head start on the growth of pea and rape. As growth tends to be selfcompounding early advantages will often lead to advantages throughout the growth season and may further be strengthened when competition for light sets in. However, conditions relating to the growth environment or cropping strategy may alter this picture. The soil nitrogen availability and cropping density had great impact on the relative competitive strength of the studied crops. At low soil nitrogen availability the pea crop had a great advantage and as cropping density was increased the pea crop became increasingly dominant. Under the given cropping condition, increased sulphur availability had very limited impact on the competitive dynamics of intercropped pea and barley.
Intercrop research has to a great extent used an experimental design that includes two crops grown as sole crops and in proportional mixtures that relate directly to sole crops (the proportional replacement design). This design has been the topic of much discussion. It is a simple design that has been valuable with respect to showing that intercrops may be more resource use efficient and productive than comparable sole crops. However if the aim is to understand underlying mechanisms in order to construct the most “optimal” mixtures then it may not always be the most appropriate design. Experimental designs that include different crop densities and proportional mixtures of crop components (response surface design) could in that respect be valuable.
In most intercrop studies conclusion about the relative competitive strength of crops are drawn on the basis of data from one final harvest which, limits the possibilities of pointing at specific structuring factors. This project shows that collecting data from several harvests throughout a growth season gave valuable indications of how competitive hierarchies were established and changed over time.